The work-energy theorem states that the net work done on a body equals:

Work-energy theorem: $W_{\text{net}} = \Delta KE = KE_{\text{final}} - KE_{\text{initial}}$. This is a direct consequence of Newton's second law. Apply $F = ma$ and integrate over displacement: $\int F \, dx = \int m a \, dx = \int m v \, dv = \tfrac{1}{2} m v^2 \big|_{u}^{v} = \Delta KE$. If only conservative forces act, this is equivalent to saying $\Delta KE + \Delta PE = 0$, the conservation of mechanical energy. With friction or other dissipative forces present, energy is lost to heat and the strict mechanical-energy conservation breaks, but the work-energy theorem still holds because work includes *all* forces.